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Grenville
The Grenville orogeny was a long-lived mountain-building event associated with the assembly of the . Its record is a prominent belt which spans a significant portion of the North American continent, from to , as well as to . Grenville orogenic crust of mid-late Mesoproterozoic age (c. 1250–980 ) is found worldwide, but generally only events which occurred on the southern and eastern margins of are recognized under the "Grenville" name. These orogenic events are also known as the in Africa and the in . Timescale The problem of timing the Grenville orogeny is an area of some contention today. The timescale outlined in Toby Rivers' recent work is derived from the well-preserved Grenville Province, and represents one of the most detailed records of the orogeny. This classification considers the classical Grenville designation to cover two separate orogenic cycles; the Rigolet, Ottawan and Shawingian orogenies compose the Grenville Cycle, and the Elzevirian orogeny stands on its own. Due to the great size of the area affected by Grenville events, there is some variance in timing across the orogenic belt. Regional variations below discusses local deviations from Rivers’ timeline, presented here. Ages are approximated from the magmatic activity associated with the individual cycles of the orogeny. The gaps in the ages of the compression cycles and isotope analysis of hornblende, biotite, and potassium feldspar suggest that extension was occurring when compression had momentarily ceased. Rivers' 2008 paper has now examined the timing of the different periods of the orogeny and reconstructed the timeline based on the spatial and temporal metamorphism of the rocks present. According to this newer version of the timeline which is a composite of Rivers 1997 and Gower and Krogh 2002, the Elzevirian orogeny occurs from 1240 to 1220 Ma, the Shawinigan occurs from 1190 to 1140 Ma and is no longer part of the Grenville cycle, the Ottawan (now 1090–1020 Ma) and Rigolet (still 1010–980 Ma) become phases which are grouped into the Grenvillian orogeny. General tectonics Reconstruction of the events of the orogeny is ongoing, but the generally accepted view is that the eastern and southern margins of Laurentia were active margins until the beginning of continental collision. This type of (B-type) tends to emplace on or near the edge of the overriding plate in modern subduction zones, and evidence of contemporary (c. 1300–1200 Ma) island arcs can be found throughout the Grenville orogen. The of South America are considered a modern analogue. From about c. 1190–980 Ma (the actual timing varies by locality) two separate continental blocks collided with Laurentia. Both of these collision events are thought to be analogous to the collision driving modern-day growth of the range. For some time one of the blocks was believed to be the continent of Amazonia, but paleomagnetic evidence has now proven that this is not the case. These periods of thrusting and were not continuous, but rather interrupted by comparatively quiet periods, during which AMCG ( / / / ) were intruded into the . Polarities of subduction (which plate overrode which) vary by region and time. Some island arc remnants were emplaced on the Laurentian margin, and some were accreted during orogeny. Timing of these events is constrained by cross-cutting relations observed in the field as well as SHRIMP ( ) and TIMS ( ) . The first period of tectonic activity was the accretion of an island arc at some point during the Elzevirian Orogeny. Before the accretion of the island arc took place, subduction between a continental plate and presumably an oceanic plate was taking place. Slab pull and far-field drivers such as ridge push were aiding in closing the distance between the island arc and the continent. Depending on the angle of subduction deformation of the continental crust was already taking place and thickening the lithosphere. By 1.19 Ga the Elzevir back arc basin was closing. From 1.18 to 1.14 Ga extension was occurring in the area. Whether due to lithospheric cooling, also known as thermal subsidence, or the compressional activity in the area reactivated some extensional faults. The extension is marked by the isotopic ages of the previously mentioned rocks. Additionally there is the formation of sedimentary basins which means the margin was quiescent enough that sediments could accumulate. However, in some areas from 1.16 to 1.13 Ga, coeval with extension, there is evidence there was still thrusting and emplacement of terranes occurring. According to one model westward thrusting occurred from 1.12 to 1.09 Ga and then extension was the primary tectonic activity until 1.05 Ga. It was at this point that the Central Granulite Terrane was exhumed and minor magmatism occurred. The precise reason for change from compression to extension is unknown but may be the result of gravitational collapse, mantle delamination, the formation of a plume underneath a supercontinent, changes in far-field drivers on the distribution of stress, or any combination of reasons originating from the fact that our planet is dynamic. The cyclic compression and extension history of this area is similar to the . In this area of the world the Wilson Cycle would be creating the basin for the proto-Atlantic Ocean ( ) General lithology Today, the Grenville orogen is marked by northwest verging and high pressure metamorphic regimes, as well as distinctive AMCG suite magmatism. Metamorphism is commonly of and facies, that is, medium to high temperature and pressure alteration. metagabbros (very high pressure metamorphic rocks) are found in some localities, and likely represent areas of deepest burial and/or most intense collision. Throughout the orogen, these sequences of high pressure metamorphic rocks are cut by intrusive AMCG suite plutons, generally interpreted as syn- or post-tectonic. AMCG plutonism is generally associated with upwelling under thinned . This is derived from the theory that AMCG plutonism is driven by ponding of basalt at the base of the during tectonic extension. The lithosphere may be thinned either convectively or by , in which the bottom portion of the lithosphere is stripped off. Both models have been proposed for the Grenville orogeny. The Grenville orogeny can be categorized into three sections based on structure, lithology, and thermochronology. The three sections, respectively called the Gneiss Belt, Metasedimentary Belt, and the Granulite terrane are all separated by shear zones. The Gneiss Belt is made up of felsic gneisses and amphibolites that were metamorphosed in the upper amphibolite to granulite facies. Thrusting in this section was low angle, but would have the potential to increase and rotate as it continued and evolved. Shear in this area is referred to as ductile shear meaning the material was cooling and becoming solid, but still behaving viscously or plasticly. The age of this belt is approximately 1.8 to 1.18 Ga. Regional metamorphism is believed to have deformed this area at approximately 1.4 Ga and metamorphic thrusting at approximately 1.16 to 1.12 Ga. The Metasedimentary Belt is predominantly sedimentary and volcanic rocks which have undergone greenschist to granulite facies metamorphism. Subdivisions of this belt include the Bancroft, Elzevir, Sharbot Lake, and Frontenac Domains and the Adirondack Lowlands. In this belt magmatism is known to have occurred between 1.42 and 1.04 Ga depending on location. As with the Gneiss Belt, metamorphism is believed to have occurred at approximately 1.16 Ga. The Granulite Terrane is composed of meta-igneous gneisses including anorthosite massifs. Anorthosites form in plutons and are composed mostly of plagioclase. The rocks of the Grenville Province in Canada are included in this category. The oldest magmatism known in this area dates to 1.32 Ga approximately. Granulite facies metamorphism began around 1.15 Ga and continued for about 150 Ma after the onset, however the continuity of the metamorphism cannot be determined. Regional variations granites.}} It is important to separate local from large-scale tectonic history of the orogenic belt in order to understand the orogeny. For this purpose, the Grenville orogen is generally broken into four localities: the southern extent in Texas and Mexico, the , the and the well-studied Grenville Province of Canada. A portion of the orogen can be found in Scotland, but due to Scotland's proximity to the Grenville province prior to opening of the Iapetus Ocean (modern day Atlantic Ocean), the two share largely the same history. Texas and Mexico Texas and Mexico represent the southern margin of Laurentia, and likely collided with a different continent than that involved in the eastern collision. The Zapotecan Orogeny of Mexico is coeval with the later stages of the Grenville orogeny, and they are generally considered to be one and the same. Mesoproterozoic igneous protoliths (metamorphosed to facies during the orogeny) fall into two age groups in Mexico; c. 1235–1115 Ma and c. 1035–1010 Ma. Rocks of the former group bear geochemical signatures implying and provenance. The latter group represents AMCG magmatism. These AMCG rocks are somewhat anomalous throughout the Grenville orogen, there is no known orogenic event which immediately predates their emplacement. It is suggested that the regime of subduction under the Laurentian margin (currently in Texas, north of the accreted Mexican ) ended around 1230 Ma, and that polarity reversed to bring the colliding continent north, since the Llano uplift, which records the history of the Grenville in Texas, bears no evidence of arc magmatism after this time. Appalachians The contain small, isolated exposures of the Grenville orogen. The largest of these, the Long Range Inlier, comprises the of Newfoundland. Other exposures include the Shenandoah and French Broad s, which comprise the province of Virginia. Blue Ridge rocks consist of various of upper and granulite facies, intruded by charnockites and granitoid rocks. These igneous rocks were intruded in three intervals: c. 1160–1140 Ma, c. 1112 Ma, and c. 1080–1050 Ma, and are massive to weakly in texture. Adirondacks This region consists of a massive dome of Proterozoic rock on the New York-Canada border. Both the Elzevirian (c. 1250–1190 Ma) and Ottawan (c. 1080–1020 Ma) orogenic pulses are recorded in the , producing high-grade metamorphic rock. A northwest-trending high-strain separates the dome into the Highlands to the southeast and the Lowlands to the northwest. It is believed that the shear zone (the Carthage-Colton) was a transpressional boundary during the Ottawan, when the Highlands were thrust over the Lowlands. Grenville province The Grenville province is named for the village of in , and constitutes the youngest portion of the . Since the area has not undergone any regional metamorphic overprinting since the orogeny, it is considered an ideal study area for Grenville and pre-Grenville age tectonics. Hence, most of what is known about the orogeny and its processes is derived from the Grenville Province. References Category:Earth